1. Technical Field
The present invention relates to apparatus used in processing, cleansing and reconstituting blood for use by a patient, such as during surgical procedures, and more specifically to apparatus for filtering blood plasma and washing cellular components of blood to remove unwanted waste components.
2. Background Information
Most surgical procedures result in some loss of blood from associated surgical incisions. Injured patients can also often experience external and internal bleeding. If blood loss from injury or surgery is substantial, it becomes necessary to replenish lost blood through transfusion.
In many instances, it is possible to collect a patient's blood for use in replacing most or even all of the blood losses. It will be readily appreciated that blood collected from a wound or a surgical site will contain tissue fragments, lysed blood cells, and other unwanted substances. Such blood must be treated for removal of unwanted substances before it is safe for reinfusion into the patient.
The general procedure of collecting a patient's blood, cleansing it, and then returning it to the patient is sometimes referred to as autotransfusion. Where autotransfusion is possible, it is a strongly preferred way of replacing a patient's blood losses. One reason that autotransfusion is so preferred is that it avoids incompatibility problems which sometimes can occur when giving transfusions of blood obtained from someone other than the patient. Use of a patient's own blood to replace blood losses has also become increasingly important in view of issues relating to the safety of replacement blood, such as the prevalence of acquired immune deficiency syndrome (AIDS) or other diseases among blood donors in some locales. Because of these benefits, and others, autotransfusion is often the method of choice for minimizing loss of cellular blood components during diverse procedures ranging from surgery to plasma exchange therapy, and is likely to become increasingly important in the future.
The process of removing blood plasma and other unwanted substances without any cleansing of the blood is commonly referred to as plasmapheresis. Plasmapheresis has long been practiced through use of filters having a pore size large enough to pass plasma and other unwanted substances found in the blood, such as anticoagulant, toxins and components of lysed cells (which, for purposes of brevity and simplicity, shall sometimes hereinafter be referred to collectively as the "waste components" of blood), but small enough to retain intact cells, such as red blood cells, white blood cells and platelets (which shall sometimes hereinafter be referred to collectively as the "cellular components" of blood). Plasmapheresis has also been practiced through use of a centrifuge to separate plasma and other suspended waste components from the denser cellular components, and then removing the plasma and associated waste components.
Simple removal of plasma and associated waste components is not adequate to remove all waste materials associated with blood. It has been found that much more thorough cleansing of blood can occur if the cellular components are washed after the plasma is removed. U.S. Pat. No. 4,631,050 describes a process of autotransfusion utilizing a membrane for filtration to separate waste components from cellular components. That patent describes an initial filtration to remove gross debris, followed by addition of a washing solution to reconstitute the blood, and then subjecting the reconstituted mixture to another filtration step in order to remove remaining waste components. Although more effective at cleansing blood than simple plasmapheresis, there is a tendency for cellular components to clog the filter when utilizing this type of process. This is a serious problem because it seriously limits the flow rate of such a device, making it impractical for autotransfusion.
Numerous attempts were thereafter made to prevent buildup of cellular components on the filter membrane. One of the more effective solutions is set forth in U.S. Pat. No. 4,935,002, which was issued to the inventor of the present invention, and which is hereby incorporated by reference as though separately set forth herein. That patent describes a high speed filter for rapidly cleansing blood so that it can realistically be depended upon as a source of blood for autotransfusion. The plasma separator apparatus disclosed in U.S. Pat. No. 4,935,002 includes within a single housing a first filtration zone where waste components of blood are separated from the cellular components; a washing zone where a wash solution is mixed with the cellular components; and a second filtration zone where wash solution and residual waste components are separated from the cellular components. A rotating disk is used to create a shear force which prevents cellular components from building up and clogging the filter pores, and also serves to prevent clotting. Although a substantial improvement over previous apparatus for conducting plasmapheresis or for cleansing blood of unwanted waste materials, the apparatus disclosed in U.S. Pat. No. 4,935,002 could still be improved upon. For example, it was found that the apparatus of U.S. Pat. No. 4,935,002 was difficult to build within tolerances necessary to avoid unwanted mixing between the two filter zones at a reasonable cost. It was also discovered that the flow rate of blood through the device disclosed in that patent was slower than desirable and did not cleanse the cellular components of blood as effectively as desired.